ANlflVEKSAET ADDEESS OF THE PRESIDENT. 83 



depolarizing power o£ unaltered chiastolite, but has been changed 

 into an isotropic substance not yet identified. "We thus see very 

 clearly that the rock must have been exposed to such conditions 

 that mica, as such, or its constituents could wander about from 

 one point to another and collect round special centres. 



3. Slate near Granite at Wicca Pool. — This furnished me with a 

 most instructive series of specimens. In one the mica and quartz 

 have segregated in the manner just described; only, in place of 

 being detached, the concretions have grown so much that they 

 have often coalesced and thrown the residual material into irregular 

 darker patches, where it has to a considerable extent crystaUized 

 into a dirty-greenish-brown isotropic mineral, which I have not 

 been able to identify, unless, indeed, it be garnet, which is 

 doubtful. 



From such a specimen we gradually pass to cases where this 

 mineral has been thrown of£ and collected, not so much into irregular 

 patches as into small thin lenticular masses lying, roughly speak- 

 ing, in the plane of stratification, so as to produce a true foliation. 

 Here and there are also lenticular masses of quartz, crystallized in 

 situ, in grains varying up to j^ of an inch in diameter. In a 

 third specimen this kind of foliation is still more perfectly deve- 

 loped, and the isotropic mineral is, to a considerable extent, repre- 

 sented, if not actually replaced, by mica of darker colour than that 

 in the rest of the rock ; and, in fact, here and there it almost dis- 

 appears. In a fourth specimen the isotropic mineral appears to 

 be absent, and the rock is composed of quartz and of very dark 

 and colourless mica, all crystallized in situ. In some parts of the 

 thin section the material was apparently uniformly stratified in 

 well-marked thin beds, and there are no transverse cleavage-joints. 

 Here the crystals of mica lie, to a great extent, more or less par- 

 allel to the plane of stratification. That they were not deposited 

 in that position as mud, however, is well shown by the structure 

 of other parts of the section, where the rock had given way to 

 lateral pressure by the formation of close, irregular, roughly par- 

 allel joints, in the manner described when treating on cleavage. 

 Here the crystals of dark mica lie along the joints, in the same 

 manner as they occur in the detached transverse cracks in speci- 

 mens of altered slate from Liskeard and Ivybridge ; so that we 

 have a good illustration of one particular type of cleavage-foliation, 

 more highly developed in some of the coarser-grained mica-schists 

 described further on. 



Another type of imperfect cleavage-foliation is when the crystal- 

 lization is modified by the presence of a true, well-developed, 

 structural cleavage. The slate of Birnam, already described, is an 

 illustration of this ; for in it we see that lenticular portions of the 

 green laminar mineral have been formed in the plane of cleavage ; 

 and in the thin beds which contain much of this mineral we have 

 a perfect, true cleavage-foliation, in which the laminae lie in the 

 plane of cleavage. A similar but finer-grained structure occurs in 

 a few of the green slates of Westmoreland. 



